I wonder if he would even be able to claim loyalty from the French army. On the one hand, Louis XVIII might well have made himself unpopular and the army nostalgic, but on the other hand there would have been a turnover and a lot of the men probably would have joined since Napoleon's exile.
I suppose it depends how much he remains in the French consciousness in his role as part of the European diplomatic system as Emperor of Elba. As I mentioned before, it's very different to his distant exile on St Helena.